Colloidal nanocrystals (NCs) are considered to be highly valuable in variety of applications. For example, semiconductor nanocrystals, also known as quantum dots (QDs), are considered as a promising candidate to be applied in the areas of solar cells, photocatalysis, and biological imaging due to their attractive properties including higher molar extinction coefficients and better photo stability compared to organic molecular fluorophores, and rationally tunable emission wavelengths. The most widely used synthetic procedure for QDs is the hot-injection method, in which molecular precursors are combined at high temperature, initiating a nucleation and growth process in which polydispersity is minimized by intentionally quenching the reaction before all of the precursors are consumed. The resulting QDs are coated with a layer of surfactant molecules (ligands, also “caps”) that provides charge balance and colloidal stability, but as-synthesized samples also contain unreacted precursors as well as reaction byproducts, high-boiling solvent(s), and/or an excess of ancillary ligands added to control growth and improve stability. Applications almost universally require purification and/or surface modification of the as-synthesized QDs: (1) for optical applications, the as-synthesized QDs are not very bright, which requires the formation of an inorganic shell to increase the quantum yield (QY); (2) for bio-imaging applications, surface modification by encapsulation or ligand exchange is essential for water solubility; (3) for electronic applications, excess ligands adsorbed on the surface hinder the charge transfer between the QDs and receiving substrates. These examples demonstrate that effective means for the isolation of NCs with well-defined surface properties is essential to the applications of NCs in solution or assembled into matrices, and is also a necessary condition for the development of sequential preparative chemistry for NC-based structures.
The traditional method for purification of NCs is a sequential precipitation and redissolution (PR) process. For the frequent case of NCs sterically stabilized by ligands with long hydrocarbon tails in nonpolar phase (e.g. hexane, toluene, chloroform or tetrahydrofuran), flocculation of NCs is achieved by introducing anti-solvents (e.g. acetone, methanol, isopropanol) to increase the polarity of the solvent mixture. Impurities that remain soluble can then be decanted away, and the NCs redissolved in a suitable solvent.
While the PR method is convenient and scalable, it carries several limitations. Fundamentally, the separation is based on solubility; for differently-prepared batches of NCs the necessary precipitation conditions are not identical since the intermolecular forces governing the solubility of the as-synthesized NCs are not inherently controlled properties. Some impurities may have solubility properties similar to the NCs, such that multiple PR cycles are necessary for complete removal. From a practical perspective, in some cases, the amounts of polar anti-solvents that are used are not tightly controlled, but even if these procedures are performed systematically, the turbidity that is considered to represent adequate precipitation of the NCs is often a subjectively determined parameter; this can lead to run-to-run variability and present difficulties in adequately describing procedures in literature. An important consideration for any NC purification method is the effect not only on the amounts of unbound species remaining in the sample, but also the effect on the number and type of bound ligands that terminate the NC surface. In the case of the PR method, the introduction of a foreign solvent can perturb the NC surface by displacing native ligands, as has recently been reported for the case of NCs purified by PR with methanol as the anti-solvent.
The strong dependence of the photophysical properties and chemical reactivity of NCs on the surface ligand population has helped to motivate increasing interest in alternative nanoparticle purification methods. Methods including extraction processes, ultracentrifugation and electrophoretic deposition have been established to purify the as-synthesized NCs in non-polar solutions. However, these methods come with a phase transfer during the NCs purification process, require specialized equipment, and present limitations in scalability.
As such, a need exists for an improved method for purification of NCs. The relatively large size of NCs compared to small molecules makes size-exclusion chromatography (SEC) an attractive technique. However, until now size-exclusion chromatography has not been described as a method to isolate natively-capped colloidal NCs in organic solution as the basis for further manipulations, nor has its efficacy as a purification technique been compared directly to precipitation-based methods.